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clusters.html
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clusters.html
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Clusters: HarfBuzz Manual</title>
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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="clusters"></a>Clusters</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="clusters.html#clusters-and-shaping">Clusters and shaping</a></span></dt>
<dt><span class="section"><a href="working-with-harfbuzz-clusters.html">Working with HarfBuzz clusters</a></span></dt>
<dt><span class="section"><a href="a-clustering-example-for-levels-0-and-1.html">A clustering example for levels 0 and 1</a></span></dt>
<dt><span class="section"><a href="reordering-in-levels-0-and-1.html">Reordering in levels 0 and 1</a></span></dt>
<dt><span class="section"><a href="the-distinction-between-levels-0-and-1.html">The distinction between levels 0 and 1</a></span></dt>
<dt><span class="section"><a href="level-2.html">Level 2</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="level-2.html#ligatures-with-combining-marks-in-level-2">Ligatures with combining marks in level 2</a></span></dt>
<dt><span class="section"><a href="level-2.html#reordering-in-level-2">Reordering in level 2</a></span></dt>
<dt><span class="section"><a href="level-2.html#other-considerations-in-level-2">Other considerations in level 2</a></span></dt>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="clusters-and-shaping"></a>Clusters and shaping</h2></div></div></div>
<p>
In text shaping, a <span class="emphasis"><em>cluster</em></span> is a sequence of
characters that needs to be treated as a single, indivisible
unit. A single letter or symbol can be a cluster of its
own. Other clusters correspond to longer subsequences of the
input code points — such as a ligature or conjunct form
— and require the shaper to ensure that the cluster is not
broken during the shaping process.
</p>
<p>
A cluster is distinct from a <span class="emphasis"><em>grapheme</em></span>,
which is the smallest unit of meaning in a writing system or
script.
</p>
<p>
The definitions of the two terms are similar. However, clusters
are only relevant for script shaping and glyph layout. In
contrast, graphemes are a property of the underlying script, and
are of interest when client programs implement orthographic
or linguistic functionality.
</p>
<p>
For example, two individual letters are often two separate
graphemes. When two letters form a ligature, however, they
combine into a single glyph. They are then part of the same
cluster and are treated as a unit by the shaping engine —
even though the two original, underlying letters remain separate
graphemes.
</p>
<p>
HarfBuzz is concerned with clusters, <span class="emphasis"><em>not</em></span>
with graphemes — although client programs using HarfBuzz
may still care about graphemes for other reasons from time to time.
</p>
<p>
During the shaping process, there are several shaping operations
that may merge adjacent characters (for example, when two code
points form a ligature or a conjunct form and are replaced by a
single glyph) or split one character into several (for example,
when decomposing a code point through the
<code class="literal">ccmp</code> feature). Operations like these alter
clusters; HarfBuzz tracks the changes to ensure that no clusters
get lost or broken during shaping.
</p>
<p>
HarfBuzz records cluster information independently from how
shaping operations affect the individual glyphs returned in an
output buffer. Consequently, a client program using HarfBuzz can
utilize the cluster information to implement features such as:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
Correctly positioning the cursor within a shaped text run,
even when characters have formed ligatures, composed or
decomposed, reordered, or undergone other shaping operations.
</p></li>
<li class="listitem"><p>
Correctly highlighting a text selection that includes some,
but not all, of the characters in a word.
</p></li>
<li class="listitem"><p>
Applying text attributes (such as color or underlining) to
part, but not all, of a word.
</p></li>
<li class="listitem"><p>
Generating output document formats (such as PDF) with
embedded text that can be fully extracted.
</p></li>
<li class="listitem"><p>
Determining the mapping between input characters and output
glyphs, such as which glyphs are ligatures.
</p></li>
<li class="listitem"><p>
Performing line-breaking, justification, and other
line-level or paragraph-level operations that must be done
after shaping is complete, but which require examining
character-level properties.
</p></li>
</ul></div>
</div>
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